The present invention relates to a process for isomerizing alkylaromatic hydrocarbons.
It is often desirable to convert one alkylaromatic hydrocarbon to a more valuable isomer thereof. For example, it is often desired to convert ethylbenzene and metaxylene into paraxylene and/or orthoxylene. Processes for producing particular xylene isomers from C.sub.8 alkylaromatic feedstocks are well known. Typically, a selected xylene isomer is recovered from a petroleum fraction, such as reformate, which is rich in C.sub.8 alkylaromatics, by fractionation, crystallization or a molecular sieve-type separation operation. After the selected isomer has been removed from the fraction, the C.sub.8 alkylaromatic residue is typically treated in a C.sub.8 alkylaromatic isomerization operation in order to form additional amounts of the selected isomer. The newly formed amounts of the desired isomer are then recovered from the isomerate by the same type separation operation used with the original petroleum fraction. In converting the various C.sub.8 alkylaromatic isomers, it has been found that ethylbenzene is relatively difficult to convert to xylene as compared to the relatively easy conversion of one xylen isomer to another. Prior art has acknowledged the difficulty of converting ethylbenzene, and the problem of ethylbenzene buildup in isomerization-separation systems has been an economic and technical drawback in many isomerization operations.
Various isomerization catalysts and flow schemes have been suggested by the art in attempting to provide efficient isomerization and isomer recovery systems for producing a selected C.sub.8 alkylaromatic isomer. For example, U.S. Pat. No. 25,753 discloses a two-stage process for isomerizing xylenes. In the first stage, a xylene, or a nonequilibrium mixture of xylenes, is contacted with a hydrogenation-dehydrogenation catalyst under hydrogenation conditions to convert a large proportion (10-35%) of the xylenes in the feed to naphthenes. In the second stage, the naphthenes formed in the first stage are contacted with a hydrogenation-dehydrogenation catalyst under dehydrogenation conditions to reconvert the naphthenes to xylenes and simultaneously to isomerize the xylenes during the dehydrogenation. One catalyst described as useful in the process is platinum on alumina or silica-alumina.
U.S. Pat. No. 3,078,318 describes the isomerization of a xylene or nonequilibrium mixture of xylenes with a platinum-halogen-alumina catalyst in a hydrogen atmosphere at 700.degree.-1100.degree. F and 1-1500 atmospheres pressure. A selected xylene isomer is separated from the isomerization reactor effluent and the residue from the isomer-separation step is recycled to the isomerization step.
U.S. Pat. No. 3,381,048 describes a process for isomerization of a xylene isomer or a nonequilibrium mixture of xylene isomers using a platinum-halogen-alumina catalyst. In the process, the water content of the hydrocarbon feed to the isomerization reactor is kept at 20-200 ppm.
U.S. Pat. No. 3,538,173 describes a process for isomerizing xylenes in which ethylbenzene in a C.sub.8 alkylaromatic-rich stream is isomerized to xylenes by controlling the C.sub.8 napthenes content in the feed introduced into the isomerization reactor to keep the C.sub.8 naphthenes content of the feed at 2-9 weight percent of the C.sub.8 alkylaromatic content of the feed. A platinum-halogen-alumina catalyst is employed in the isomerization reactor at a temperature of 700.degree.-840.degree. F and a pressure of 3-20 atmospheres. U.S. Pat. No. 3,553,276 describes a process for isomerizing xylenes in which, during recovery of a selected xylene isomer from the isomerization reactor effluent, loss of C.sub.8 naphthenes from the system is minimized by maintaining a high concentration of diluent toluene in the effluent from the isomerization reactor. The retention of C.sub.8 naphthenes is accomplished by introducing large amounts of diluent toluene into the isomerization reactor in the feed. A platinum-halogen-alumina catalyst is used in the isomerization step at a temperature of 30.degree.-1290.degree. F and a pressure of 1-100 atmospheres, or more.
U.S. Pat. No. 3,879,484 describes a process for isomerizing C.sub.8 alkylaromatic hydrocarbons, such as xylenes, by contacting the C.sub.8 alkylaromatic hydrocarbons with a platinum-rhenium-halogen alumina catalyst at a temperature of 30.degree.-1112.degree. F and a pressure of 1-100 atmospheres; see also U.S. Pat. No. 3,577,475.
Activity and stability are important properties of an isomerization catalyst. One measure of activity is the capacity of a catalyst to provide sufficient conversion at any given operating temperature to achieve a close approach to equilibrium concentrations of isomers in the product. Stability refers to the ability of a catalyst to maintain a desired level of activity over an extended period of use without the need for excessively increasing the operating temperature. Typically, when a catalyst begins to lose activity, the operating temperature of the isomerization process is increased to maintain the desired activity level. A stable catalyst requires only a relatively slow temperature increase, while a relatively less stable catalyst requires a more rapid increase in temperature to maintain the same activity level.
In a C.sub.8 alkylaromatics isomerization system for producing paraxylene and/or orthoxylene with a catalyst containing platinum and halogen, the temperature is typically raised at a constant rate, or stepwise, to maintain catalyst activity at a given level. It has been found necessary, when the temperature is thus raised, to likewise raise the hydrogen pressure in the isomerization system simultaneously to maintain an acceptable level of conversion of ethylbenzene to xylenes. An increase in hydrogen pressure in the isomerization system causes an increase in saturation of C.sub.8 alkylaromatics in the feed to form C.sub.8 naphthenes, i.e., the selectivity of the catalyst for isomerization is reduced by increasing the hydrogen pressure. The formation of excessive amounts of C.sub.8 naphthenes is undesirable because it (1) consumes hydrogen and (2) consumes C.sub.8 alkylaromatic hydrocarbons. This necessitates addition of undesirably large amounts of expensive hydrogen to the system and also reduces the potential C.sub.8 alkylaromatic product isomer yield. Thus, it is apparent that the stability of an isomerization catalyst is important to economical operation of an isomerization system because it allows the system to operate at a lower temperature for a longer time, thereby providing greater overall catalyst selectivity.
In an embodiment, the present invention relates to an improved process for isomerizing an alkylaromatic hydrocarbon by contacting a feed including the hydrocarbon and hydrogen with a catalyst including 0.01-3 weight percent platinum and 0.01-3 weight percent rhenium on an alumina support at isomerization conditions including a temperature of 700.degree. F to 900.degree. F and a hydrogen pressure between 100 psi and 300 psi, the improvement comprising increasing the activity and selectivity of the catalyst by the method comprising: including in the catalyst greater than 1.2 weight percent combined chloride and contacting the feed with the catalyst in the presence of between 1.5 and 150 ppm, by volume, of free chloride and not more than 10 ppm, by volume of water, based on the volume of the feed.
I have found that a particularly effective process for isomerizing alkylaromatic hydrocarbons is obtained by employing particular isomerization conditions in combination with a particular isomerization catalyst. The feed is contacted with the catalyst in the presence of 1.5-150 ppm free chloride under very dry conditions, in the presence of not more than 10 ppm, and preferably below 1 ppm, water. The catalyst employed in the process of the invention is a platinum-rhenium-alumina composition which has a combined chloride content adjusted to above 1.2 weight percent and preferably about 1.5 weight percent. When the above-described isomerization conditions are used in conjunction with the high cloride platinum-rhenium catalyst, a particularly active, selective and stable isomerization system is achieved.